I would stay with this somewhat longer before even considering a shop. Fully realising it is currently frustating. Getting that exhaust loosened off may tell something. I do not own a 603 but if the front cover has the factory set timing indicator. It was adjustable for factory manufacturing tollerance reasons.

Since you changed it out. You may have to do the tdc fluid check to see if it is calabrated the same. Although only a few degrees off should not display symptoms I would think as bad as you seem to have.